1. Field of the Invention
The present invention relates to an improvement of a coupling circuit using a photocoupler. More particularly, it relates to a coupling circuit having means for preventing erroneous operation caused by a surge voltage etc.
2. Description of the Prior Art
A solid state relay arranged in high density to electrically separate an outer signal source from a circuit for a driving control of a load by coupling with a photocoupler has been well-known in such as U.S. Pat. No. 3,723,769.
Thus, in the solid state relay arranged in high density, the parts of the circuit in the input side are near the parts of the circuit in the output side whereby a stray capacitance is high not to be negligible. When high surge voltage is applied between the input side and the output side, the reverse breakdown of the light-receiving element for the photocoupler is caused to pass a leak current from the circuit in the output side through the stray capacitance to the circuit in the input side. Even though the surge voltage is raised, the light-receiving element is in the ON state whereby the transistor or the other element of the circuit in the output side disadvantageously causes the erroneous operation.